1. Field of the Invention
This invention relates to biodegradable films, particularly, those useful for agricultural mulch and heat-sealable packaging films that are stable to weathering conditions for a predetermined period and then decompose.
Applying plastic film for agricultural mulching has become an important technique for increasing the yield and quality of vegetable and fruit crops and for decreasing the production costs of these crops. Principal benefits of mulching are to provide weed control, warm the soil for early crop production, control soil moisture, and reduce nutrient leaching.
The packaging of various items in plastic films has become an important factor in the safe and economical distribution and use of many consumer products.
2. Description of the Prior Art
Polyethylene is the most common resin used for making mulch and packaging films. However, when used as a mulch, it must be removed from the field and burned or buried at the end of each fruiting season since it does not decompose in time to start the subsequent crop. In fact, the film persists for several years if not removed. Because the removal and burying or burning of mulch film is both costly and has an adverse effect on the ecology, the need for a plastic mulch that will decompose by the end of a growing season has recently become apparent. Likewise, the buildup and pollution caused by plastic packaging films in municipal landfill and disposal areas could be reduced if such films were degradable.
Numerous attempts have been made to produce degradable films from petroleum and cellulose derived materials (Chemical Week 109: 45-46 (1971) including polyethylene-coated paper (Chemical Week 110: 44 (1972) and polybutene-1 films (U.S. Pat. No. 3,590,528). None has been completely successful, apparently because they are too costly or they decompose too slowly for many applications.
Although whole starch has been investigated for many years as a potential raw material for nonsupported films, it has never achieved large-scale commercial success because its films are brittle and are greatly affected by moisture (Cereal Chemistry 40: 154-161 (1963). Large amounts of compatible water-soluble plasticizers, such as glycerol, are effective softening agents for whole starch, but the resulting dried films are too soft and tacky at high humidity and have virtually no wet strength. Also, upon contact with water, the plasticizers are readily leached out leaving a brittle product upon drying. A solution to the tackiness and wet strength problem is disclosed by Otey et al. in U.S. Pat. No. 3,949,145 wherein a starch-polyvinyl alcohol glycerol film is coated with a water-resistant coating. However, the application of coatings is expensive and does not completely prevent the leaching of plasticizers. In the area of packaging, starch-based films heretofore known in the prior art have not been heat-sealable.